1. Field of the Invention
The present invention relates generally to an apparatus for separating carbon dioxide from combustion gas and, more particularly, to an apparatus for efficiently separating carbon dioxide from combustion gas using multiple separation membranes and for allowing easy application to an actual process.
2. Description of the Related Art
Generally, technologies for capturing carbon dioxide that contributes to greenhouse effects have been developed recent years. The technologies for capturing carbon dioxide can be classified into post-combustion capture, pre-combustion capture, and oxyfuel combustion capture. Post-combustion capture can be classified into chemical absorption using an aqueous amine or ammonia absorbent, dry-type absorption using a solid absorbent instead of an existing aqueous absorbent, and a membrane separation using a separation membrane, etc. The membrane separation method has an advantage of being an environmentally friendly process. However, only membrane materials and modules have been mainly developed, and studies on post-combustion capture have been conducted by only a minority of companies.
U.S. Pat. No. 7,964,020 discloses gas separation processes using membranes with permeate sweep to remove carbon dioxide from combustion gases.
U.S. Pat. No. 7,964,020 includes processes of injecting a flue gas stream to be treated into a feed side of a membrane, injecting sweep gas, usually air, to a permeate side of the membrane, and passing a permeate stream and the sweep gas to a combustor. In this way, the permeate stream of the membrane is withdrawn to a boiler with the sweep gas, thereby building up carbon dioxide concentration on the feed side of the membrane. However, the above processes disclosed in U.S. Pat. No. 7,964,020 have a limited application to an actual process for a power plant since the processes cause a change in efficiency of the boiler.
U.S. Pat. No. 4,264,338 discloses a method for separating gases by a process using multiple stages of membrane separation, whereby an unpermeated residue of a second stage membrane or a third stage membrane is recovered, and a permeant mixture is passed to an inlet of a first stage membrane and therefore a separation factor of the first stage membrane is increased. However, the gas separation method recovering and using an unpermeated residue has a limitation in increasing carbon dioxide separation performance of a gas separation system.